1. Field of the Invention
This invention relates to a liquid withdrawing tray for a purification column, a purification column, and a method for purification. More particularly, it relates to a tray for withdrawing a liquid from a middle portion or middle level of a purification column, a purification column furnished with the tray, and a method for purification an easily polymerizable compound such as (meth)acrylic acid by the use of the purification column.
2. Description of the Prior Art
As a method for production of acrylic acid and esters thereof, the method by oxidizing propylene and/or acrolein with a molecular oxygen-containing gas in vapor phase, recovering a reaction product gas thus obtained with a solvent, and subjecting a reaction product-containing solution thus obtained purification in a purification column to obtain purified acrylic acid or esters thereof (JP-A-9-227445).
In the purification column, such operations as withdrawing reflux liquid in the column from the middle portion, generally the middle level, of the column for the purpose of withdrawing an intermediate product, and cooling or heating the reflux liquid are routine. Then, for the purpose of withdrawing the liquid from the middle portion of the purification column, the chimney tray which is composed of a vapor conduit (gas riser), an umbrella covering the conduit, and a liquid withdrawing tube has been popularly used (refer, for example, to Henry Z. Kister, xe2x80x9cDistillation Operation,xe2x80x9d pp. 102-116 [published in 1990 by McGraw Hill, Inc.]). The chimney tray which is concurrently provided with a vapor conduit and a liquid withdrawing tube is also available for the purpose of the withdrawal of interest.
In purification of such an easily polymerizable compound as (meth)acrylic acid, the measure taken to polymerize the easily polymerizable compound is important for operating the purification column stably. For inhibiting this polymerization, the practice of adding such a polymerization inhibitor as phenothiazine, hydroquinone, or methoquinone has been in vogue.
When the purification column provided with the chimney tray is used in purifying the easily polymerizable compound, however, the polymerization of the compound occurs throughout the entire chimney tray inclusive of the vapor conduit and the umbrella in spite of the use of the polymerization inhibitor and the polymer brings the problem of clogging the chimney tray and preventing it from being stably operated for a long time. Then, in the case of the chimney tray provided concurrently with a vapor conduit and a liquid withdrawing tube, since the descending liquid comes into counterflow contact with the vapor in motion at a relatively high speed, this vapor entrains liquid splash to the upper part of the purification column and impairs the. separating effect of the purification column. This impairment possibly results in increasing the content of impurities in the target product of purification obtained via the top of the column.
An object of this invention, therefore, is to provide a novel liquid withdrawing tray for the purification column, a purification column utilizing the tray, and a method for purification.
Another object of this invention is to provide a tray for withdrawing the liquid from the middle portion or middle level of the purification column, a purification column provided with the tray, and a method for purification such an easily polymerizable compound as (meth)acrylic acid by the use of this purification column.
Still another object of this invention is to provide, in purification such an easily polymerizable compound as (meth)acrylic acid, a novel liquid withdrawing tray adapted to allow effective prevention of the polymerization and enable the purification to be performed stably for a long time, a purification column provided with the tray, and a method for purifying the easily polymerizable compound by using the purification column.
The objects mentioned above are accomplished by the following items (1)-(10).
(1) A liquid withdrawing tray for a purification column, comprising a liquid receiving plate and a liquid withdrawing tube communicating with the liquid receiving plate and serving the purpose of withdrawing the liquid.
(2) A tray set forth in (1) above, wherein the receiving plate has the shape of a funnel.
(3) A tray set forth in (1) or (2) above, wherein the liquid receiving plate is provided in the upper outer periphery thereof with notches.
(4) A tray set forth in any one of (1)-(3) above, wherein the angle of the downward slope of the liquid withdrawing tube from the horizontal direction is in the range of 0.5xc2x0-80xc2x0.
(5) A purification column provided in the column interior thereof with a liquid withdrawing tray set forth in any one of (1)-(4) above.
(6) A purification column set forth in (5) above, wherein the largest cross sectional area, S1, of the liquid withdrawing tray is in the range of 10-90% of the cross sectional area, S2, of the purification column.
(7) A purification column set forth in (5) or (6) above, wherein the liquid receiving plate is provided thereunder with at least one spray nozzle for spraying the lower side of the liquid receiving plate with a liquid.
(8) A purification column set forth in any one of (5)-(7) above, wherein the liquid receiving plate is provided thereover with at least one liquid injecting nozzle for injecting a liquid into the liquid surface in the liquid receiving plate.
(9) A method for purifying an easily polymerizable compound, which comprises effecting the purification by the use of a purification column set forth in any one of (5)-(8) above.
(10) A method set forth in (9) above, wherein the easily polymerizable compound is at least one member selected from the group consisting of (meth)acrylic acid and esters thereof.
In the purification column, the polymerization inhibitor is present mainly in the liquid phase and hardly ever in the gas phase. When the vapor in the gas phase condenses in the dry part of the interior of the purification column, therefore, the condensate readily polymerizes because it is exposed in a state destitute of the polymerization inhibitor at a relatively high temperature in the purification column.
In the case of the chimney tray, the lower (reverse) surface of the umbrella covering the vapor conduit is not always kept wet with the condensate and, therefore, is possibly suffered to permit the appearance of a dry part. More often than not, the polymerization occurs in the dry part under the umbrella.
The study further continued has brought a discovery that the use of a funnel-shaped tray devoid of an umbrella in the place of the conventional chimney tray deprives the tray surface of the dry part and consequently permits effective prevention of the occurrence of the polymerization. The present invention has been perfected on the basis of this knowledge.
By adopting the liquid withdrawing tray of this invention and using the purification column provided with this tray in purifying an easily polymerizable compound such as (meth)acrylic acid, for example, it is possible to effectively prevent polymerization in the tray of easily polymerizable compounds.